There exists a valve device constituting an airflow passage of a fuel tank and having a float chamber at a lower side and an airflow chamber at an upper side, in which a port edge at the lower side of the airflow port provided in a separation wall for separating the float chamber and the airflow chamber is defined as a valve seat for the float valve, and a port edge at the upper side of the airflow port is defined as a valve seat for a check valve accommodated in the airflow chamber (see Patent Literature 1).
For this type of valve device, it is necessary to change the size of the airflow port in order to change and adjust flow rate characteristics and the open valve pressure. In this case, in the case of the valve device described in Patent Literature 1, it is necessary to replace the entire case main body having the airflow port in order to change and adjust the flow rate characteristics and the like.